1. Field of the Invention
The present invention relates to electronic overload protection devices for circuits containing solid state power components. More particularly, the present invention relates to overload protection devices for power output circuits having a unity voltage gain.
2. Description of the Prior Art
A hitherto known protection device is exemplified by the teachings of Italian Pat. No. 958,867, corresponding to U.S. Pat. No. 3,792,316. According to the teachings of this prior art patent, in order to protect an audio amplifier final stage output transistor, both the current passing through the transistor and the voltage between its collector and emitter terminals are measured. The measured values of the current and voltage thus obtained, whose product corresponds to the instantaneous power dissipated by the transistor, are supplied to a protective circuit connected to the control stage of the transistor. When the measured instantaneous power due to an accidental overload, (e.g.--a short circuit at the output), reaches a predetermined level that could destroy the output transistor, the protective circuit operates the control stage so as to limit the input signal to the transistor, thereby maintaining the output transistor's dissipated power within the safety limits defined by the characteristics of the transistor.
The aforementioned type of prior art device is very suitable for preventing the destruction of the power components of a power output circuit due to an accidental overload but cannot prevent such components from operating at their maximum rated power for the entire period when overload conditions prevail. Such maximum rated power operation of the power output circuit results in a high consumption of energy, which causes overheating of the power output circuit of which the protected power output transistor is a constituent part. In addition, the prolonged continuous operation of the power output circuit at maximum rated power may cause serious damage to the structures adjacent thereto. A typical example where prior art protection devices have proved to be inadequate is in the design of protection devices for use in car radio power output circuits. An accidental short circuit of the power output stage may cause a rapid discharge of the car battery to which the car radio is connected and may result in the burning or destruction of the plastic components adjacent thereto.